With higher performance achieved in mobile devices such as cell phones, laptop computers, and digital cameras, being anticipated for the power source is the practical use of fuel cells using a solid polymer electrolyte membrane. Among solid polymer fuel cells (hereinafter, simply referred to as “fuel cell”), direct oxidation fuel cells, in which fuel such as methanol is directly supplied to the anode, are well-suited in terms of reducing size and weight, and thus, their development as a power source for mobile devices is currently underway.
A fuel cell is provided with a membrane electrode assembly (MEA). The membrane electrode assembly is composed of: a polymer electrolyte membrane; and an anode (a fuel electrode) and a cathode (an air electrode) that are bonded to each side thereof, respectively. The anode is composed of an anode catalyst layer and an anode diffusion layer, and the cathode is composed of a cathode catalyst layer and a cathode diffusion layer. The MEA is interposed between a pair of separators, thus constituting a cell. An anode-side separator has a fuel flow channel for supplying fuel such as hydrogen and methanol to the anode. A cathode-side separator has an oxidant flow channel for supplying oxidant such as oxygen and air to the cathode.
There are some goals with respect to putting a direct oxidation fuel cell into practical use.
First of these is long life characteristics. In a fuel cell, output power gradually becomes lower as time elapses for power generation. For example, a power source for a mobile device is required to maintain output power for a total of 5000 hours or more; currently however, long life characteristics have not yet been achieved to such an extent.
There are some factors that cause output power degradation. First of these is a phenomenon called methanol crossover (MCO) in which fuel such as methanol supplied to the anode permeates through the electrolyte membrane and travels to the cathode. Since MCO lowers the cathode potential, output power of the fuel cell degrades. Also, methanol that has reached the cathode after permeating through the electrolyte membrane then reacts with air. This reaction causes excessive air consumption leading to air deficiency at the downstream side, and thus causes output power of the fuel cell to degrade. The amount of MCO tends to increase as time for power generation elapses, and is assumed to affect life characteristics.
In order to reduce MCO, reducing methanol diffusivity in the anode diffusion layer is considered effective. However, if methanol diffusivity is reduced in the entire anode, methanol becomes deficient at the downstream side of the fuel flow, thus causing output power to degrade.
In view of the problems mentioned above, a proposal is made to increase the methanol permeability coefficient of the anode diffusion layer so that the more downstream the fuel flow, the larger the coefficient (Patent Document 1). This enables the supply amount of methanol to be secured at the downstream side of the fuel flow, while also reducing MCO at the upstream side thereof. Specifically, a proposal is made to allow change in the composition and the thickness of the conductive water repellent layer included in the anode diffusion layer, at the upstream side and the downstream side of the fuel flow. The conductive water repellent layer includes a conductive agent and a water repellent agent.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2002-110191
Patent Document 1 proposes that changes be allowed in the composition and the thickness of the conductive water repellent layer included in the anode diffusion layer; however, sufficient effects cannot be achieved by such means, and achievement has not yet been made to the extent of enabling long life characteristics that are satisfactory. There are two reasons to the above, based on the characteristics of a conductive water repellent layer.
First, a conductive water repellent layer usually has a thickness of only about 50 μm. Therefore, even if the composition and the like of the conductive water repellent layer are changed, it is difficult to significantly change the permeability of fuel such as methanol. Particularly, in the case of a high fuel concentration, the fuel permeability with respect to the conductive water repellent layer becomes entirely high. Therefore, in the case of a thin conductive water repellent layer, it is difficult to achieve an effect of changing fuel permeability.
In addition, a conductive water repellent layer serves to bond the diffusion layer and the catalyst layer together, while also serving to control fuel diffusion. If the composition and the thickness of the conductive water repellent layer change significantly, the bonding strength between the diffusion layer and the catalyst layer becomes easily deteriorated. Thus, it becomes difficult to secure conductivity, and further, to control the fuel diffusivity in the entire anode.
Given the above, an object of the present invention is to provide a direct oxidation fuel cell with reduced MCO and improved long life characteristics even when using an aqueous methanol solution containing a high concentration of methanol which is highly diffusive, by controlling fuel diffusivity in the anode.